Earth May Have Made Much of Its Own Water as Rocky 'Pebble Rain' Fell Through Hydrogen
University of New Mexico experiments turned nearly 80% of the iron oxide in rock samples into iron and water vapor, suggesting hundreds of oceans' worth could form this way.
Earth may have produced much of its water while the planet was still forming, as rocky pebbles reacted with the hydrogen-rich atmosphere surrounding the young world. That is the conclusion of new research published in Nature Geoscience by a team at the University of New Mexico, and it offers an explanation for how Earth acquired one of the essential ingredients for life.
The study was led by graduate student Susmita Garai, with Zachary Sharp, professor and director of Earth and Planetary Sciences, and Peter Olson. Scientists have long debated where Earth's water came from. One leading explanation relies on delivery from far away, through comets, meteorites or icy pebbles that formed in the colder regions of the early solar system. Questions remain about how that water reached Earth and whether those processes fully explain the planet's chemistry.
The New Mexico team tested another idea. As Earth formed, small rocky particles called pebbles piled up to build the planet, a process known as pebble accretion. Once the growing planet was massive enough, it could hold a hot, high-pressure atmosphere rich in hydrogen. As silicate pebbles fell through that atmosphere, iron oxide in the rocks could react with hydrogen to produce metallic iron and water vapor.
To test it, the researchers heated iron-bearing silicate rock samples at temperatures between 1,200 and 1,400 degrees Celsius in streams of hydrogen and, for comparison, helium. The runs lasted from 15 minutes to three hours. In the hydrogen experiments, nearly 80% of the iron oxide in the samples was converted into metallic iron, producing water vapor in the process. The same reaction did not happen in the helium runs, which shows that hydrogen is the key to the water-making step.
"Previous ideas about the source of water to Earth rely on an external source, such as comets or meteorites," Sharp said. "Here we show that virtually any Earth-sized planet should have water by this newly identified mechanism." He said the experiments were very challenging to build and run, and that the team found water is produced in just minutes in the high-temperature environment of the early Earth's atmosphere.
The amount surprised even the authors. "We had suspected and hoped that we would produce iron metal and water in our reduction experiments, but we really had no way of knowing how fast and how much," Sharp said. "We were quite surprised to find that extrapolation of our work suggests that hundreds of oceans' worth of water were likely produced in this way." He added that it is hard to imagine, looking over a large body of water, that it may have been produced 4.5 billion years ago in a hell-like atmosphere, but "it very well may have."
The findings suggest water production was an important part of Earth's early development. Rather than needing all of its initial water to arrive as ice or in other water-rich material, a growing planet could generate water through reactions between its rocky building blocks and its surrounding atmosphere. The authors say the same process could be common on rocky planets beyond our solar system, which would widen the range of worlds that might start out with water.
